Fig. 3: Porous crystal-based artificial nanochannels with pore-in-pore structures.

a Schematic of KcsA K⁺ channels. b Crystalline porous organosulfonate-amidinium salts (CPOSs) and the presented channel. c A single conical PET nanochannel with assembled CPOSs inside. d Ion selectivity of the CPOS-PET nanochannel. Reproduced with permission²². CC BY (https://creativecommons.org/licenses/). e Crystal structure of crown ether (1,10-diaza-18-crown-6-ether). f Scanning electron microscope (SEM) image of a micropipette tip filled with crown ether crystal. g Schematic of ion transport through crown ether crystal channels. h The conductance of NaCl and CaCl₂. The inset displays the calculated ion selectivity. Reproduced with permission³⁹. CC BY (https://creativecommons.org/licenses/). i Structure of MIL-53-COOH MOF and its assembly in a bullet-like PET nanochannel. I‒V curves (j) and the tip-to-base ion rectification ratios (k) of the MIL-53-COOH-PET nanochannel in different electrolytes. Reproduced with permission¹⁰. CC BY 4.0 (https://creativecommons.org/licenses/by-nc/4.0/).